Separating Metals from Mud - Mining Technologies

Professor Jan Cilliers, Imperial College London

Diana - An important resource that we find in deep dark places are the metals and minerals we need for industry and everyday life, which means that mining minerals like copper and platinum is a multi-billion pound industry. Dave and Meera have been out exploring how you separate the metal we do want from the rocks we don’t...

Meera - This week, Dave and I are exploring the process of mining, so where our metals and our minerals actually come from. Dave, it’s not as simple as just digging them out of the ground, is it?

Dave - That's right. When you normally think about mining, of course you think about digging stuff out of the ground and then you learn in school about taking those ores, smelting them, and turning them into a useful metal we can use every day. However, there's a big, really important process inbetween that. The ore which we dig out of the ground is a combination of rock and the useful minerals we want. The problem is the proportion of the useful minerals can be only 1 or 2 percent, and that's still worth extracting. And this process of extracting the valuable minerals out of the rock is incredibly difficult. Originally, this was done by processes like panning, you swirl water over a mixture of sands and the densest ones fall to the bottom, but doing this on an immense scale is a very, very difficult process.

Meera - Well one scientist looking into this separation process is Professor Jan Cilliers from Imperial College London. We’ve come down to the Royal School of Mines in Central London to find out more about this.

Jan - I think maybe we should take a step back and just look at the scale of the mines we’re dealing with nowadays. The copper mines we’re talking about treat literally tens of millions of tonnes of rock a year to produce hundreds of thousands of tonnes of copper. But within the mine, maybe only 2 percent of the rock we mine is valuable mineral and the other 98 percent is waste. In order to do the separation between the valuable minerals and the waste minerals, we need to grind them very finely down to, let’s say less than a tenth of a millimetre before we can do the separation. This is called liberation. Once we’ve ground up the rocks and they're liberated from each other now, we actually want to do the separation from the valuable minerals to the non-valuable ones. The work horse of mineral processing nowadays where we have to separate out these very small amounts of minerals from each other is a technique called froth flotation which depends on the hydrophobicity differences between different minerals. In this case, we make the one mineral we do want hydrophobic, meaning water-hating, blow air through this mixture, a slurry, or almost a mud of the mixture of minerals, the hydrophobic particles stick to the bubbles, float to the surface, and form a continuously overflowing froth that contains all our valuable minerals.

Dave - So are the valuable minerals chemically different so that your additive can make them hydrophobic?

Jan - They have to be and in this case, nature has made it so, that the valuable minerals are sulphide minerals, they are copper iron sulphide, or copper sulphide, something like a chalcopyrite as it’s called versus an oxide mineral which is our gangue or our waste minerals, the other 98 percent is that. So, it’s quite straightforward for this chemical to attack the one mineral versus the other one, and make it hydrophobic, whereas it ignores the other ones and leaves them hydrophilic, or water-loving.

Meera - What is the chemical used to make them hydrophobic?

Jan - We use two chemicals commonly. The first ones are the xanthates and the second ones are the diathocarbonates, and they both are long hydrocarbon chains with a reactive front end that looks for sulphur and it attaches to the sulphur, or chemically attaches to the sulphur, leaving a long hydrocarbon chain in the solution that looks oily and makes the mineral look hydrophobic and act like it hates being in water.

Meera - So what types of minerals is this process used to extract?

Jan - Pretty much all the base metals; copper, lead, zinc primarily. Many of the other metals are associated with the base metals, so platinum comes up as a nickel-copper type deposit, silver often comes with lead, and in all the major gold mines in the world now, the gold is actually a by-product of the copper production.

Meera

Froth Flotation

Jan - At the moment, the flotation process for copper accounts for about 70 percent of the world’s production. If you look at a typical mine in North America or in Chile where the mass of copper mines are nowadays, a typical mine would maybe treat 10,000 tonness of rock an hour. That's a cube of rock 15 metres by 15 metres by 15 metres every single hour. So, the scale and the tonnage is just unimaginably large.

Meera - And the tanks themselves then that the rock is in and the bubbles are being pushed through...

Jan - The biggest tanks nowadays are 300 to 400 cubic meters each and a typical mine would have 60 to 70 of these tanks in a row. So yes, we’re talking about aircraft hangers of froth.

Meera - This process has been used for about 100 years now. We’re in your lab where you study the foam and flotation process. What aspects of this are you studying to further it or enhance it?

Jan - We’ve been looking at the physics of the froth; the bubble size, the velocity at which it flows, and what variables affect that.

Meera - We’ve got a large water bucket/tank in front of us with a gutter around it, with many tubes going in, many tubes coming out, laser monitors on top, as well as cameras attached. So, I imagine you initiate a foam here, but what are you actually monitoring and how?

Jan - As you see, the tank is containing our slurry. The froth is forming inside them, overflowing the edge into what you call the gutter or we call the launder, and then overflows into a re-circulation tank, and through the pump as you can see. What we’re measuring is two things. The one is the height of the foam overflowing the edge of the tank and the velocity which we measure with a video camera, you can see in the back there. We are trying to measure the volume of the froth overflowing, so we need a velocity and we need a height, and of course, we know what the length of the lip is. If we multiply those three together, we get the volume of the froth.

Dave - I guess the depth at which the froth is overflowing is quite important because if it hasn’t had time to separate out, and you're sort of pumping froth through too quickly, you're going to get lots of other minerals mixed in and if you go too slowly, you're just not going to get enough production from your system.

Jan - What we’ve discovered over our years of research is that the rate at which the bubbles are bursting on the top surface is the critical parameter for controlling this process.

Dave - So if the bubbles are bursting, you're essentially losing the separation which the bubbles have done already for you.

Jan - If there's too much bursting, we lose the mineral we want. If there's too little bursting, we collect too much of the minerals we don't want and it’s that delicate balance that we’re trying to optimise. The main variable to control the rate of bursting is how much air we put into the tank.

Meera - Is this something that's controlled and monitored on industrial mines at the moment?

Jan - In some mines, it is. It’s not necessarily controlled although there is move towards that. We’d like to see that implemented far more widely and that this technique of monitoring the volume of froth overflowing is used as a control measure. If you look at a typical copper mine, we recover about 90 percent of the minerals that come in to our process and we lose about 10 percent. If we push that to 91 percent, that extra percent is worth, say, $20 million per annum. So there’s big incentive for us to get this right.

Diana - What a lot of money! That was Jan Cilliers, Professor of Minerals Processing at Imperial College London, taking Meera and Dave through the copper separation process.

Comments

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Besides floating, there are several other common techniques used to separate ore from waste. Among gravity methods are spirals, bowls, jigs and tables- all of which work well on ores with high density differences. Common chemical means used on oxide ores include solvent extraction and cyanide leaching. The old Berkeley Pit in Butte, Montana (large copper mine) is now full of water and a toxic brew of minerals. Quite a bit of research is currently being done on cleaning the water and recovering minerals in solution. One successful method used is shredded steel cans (#10 cans used in restaurants)- the metal laden water is run through the shredded steel resulting in the iron being replaced by copper and other elements.
Bass, Tue, 29th Mar 2011

To expand on what Bass said, the most common means of separating metals from waste material is gravity separation. Prospectors who are out to find gold, for example, will first classify the material using screens in order to try to get the particles the same size. Gold is 19 times heavier than water and also much heavier than most of the waste material. When the particles are the same size and subjected to aeration in water, the heavier particles (gold) will sink to the bottom of the container, allowing the lighter material to wash out. The most common example of this is called "panning". Panning is one of the oldest and still the most reliable means of separating metals from waste material. There are several tools prospectors use for gravity separation. The pan, sluice, panning wheel, "blue bowl", "gold cube" and shaker table. Googling these items should help you to visualize and help understand them. With the modern day gold rush that is being driven by the high gold prices currently, there are several new designs in gold recovery products being developed, tested and implemented in the field. In all processes, the final step to clean up is still the good old gold pan. Adaquate classification and aeration is the key to success.

The floatation process is also very effective but requires all the material to be crushed (pulverized) and chemicals are added to cause the precious metals to float where it can be collected and separated from the waste material. This type of process is usally done by large mining companies with a large facility.

In the old days, mercury was used to recover very fine gold. Gold has an affinity towards mercury (it is attacted to it). Prospectors would use mercury to collect gold particles and then retort the mercury (heat it until mercury vaporizes) and the gold would be left in the container. This is a VERY dangerous process today, and using mercury to capture gold is illegal. Google gold amalgamation.

Diamonds are a bit different. They are heavy too, but not as heavy as gold. They are not as easy to separate using the gravity separation process. For diamonds, grease is implemented. Diamonds have an affinity towards grease and will stick to it. I'm not sure if this will also work for corundum minerals.
Boogie, Thu, 2nd Aug 2012

Everything Boogie said was correct except one part, using mercury to recover gold is not illegal. With the right permits this can be done legally on a large scale. There are other ways to separate gold from mercury besides vaporizing the mercury. I actually find mercury to be a very productive method for separating gold from ore concentrate. The key to this is safety for the environment and yourself. When done correctly this method proves to be the best way in my opinion.
bigblock, Fri, 3rd Aug 2012

Thanks for clearing that up, bigblock. I didn't make it very clear that I was referring to using mercury in the field to capture gold. The old timers would use copper plate and mercury to capture fine gold as mercury will stick to the copper plate, and gold will stick to the mercury. I believe that sort of use is concidered illegal.

Anyway, yes, I have some friends that clean up cons (concentrated heavy material containing gold or other precious metals.) with mercury. You really need to be cautious and know what you're doing though. I very strongly suggest that newbies avoid it unless you have someone experienced to run some cons with you first. A person can become toothless, mad, and then dead pretty quick by inhalling the vapors.

What is the name of the alternative process that doesn't involve a retort?
Boogie, Fri, 3rd Aug 2012

You can dissolve the mercury with nitric. It would leave the gold behind while the mercury is in solution. You can also recover the mercury from the nitric by adding aluminum to the solution although I cant seem to get this to work for me. I have heard of people pressing the mercury through a type of cloth which filters out the gold.

Edit: Boogie was correct about the method the old-timers used to recover gold with mercury is illegal in a open circuit. He was also correct to say that recovery with mercury should not be attempted by someone new.
bigblock, Tue, 7th Aug 2012

Most diamonds fluoresce under X-Rays, so some mines crush the ore and run it under an X-Ray tube. Cameras detect the glow, and the diamonds are separated into a different stream by blasts of air.

This sure beats one old-time method, where a diamond deposit in a sandy desert was mined by paying the locals to lie down on their stomachs in the hot sun, and sift through the sand, a handful at a time.

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